1. Field of the Invention
The present invention relates generally to medical devices and methods and, more particularly, to a programmable, microprocessor based controller and method for controlling the temperature and flow of a thermal exchange fluid that is circulated through a heat exchange catheter inserted into a patient's body for the purpose of cooling or warming at least a portion of the patient's body.
2. Description of Related Art
Under ordinary circumstances, the thermoregulatory mechanisms of a healthy human body serve to maintain the body at a constant temperature of about 37° C. (98.6° F.), a condition sometimes referred to as normothermia. To maintain normothermia, the thermoregulatory mechanisms act so that heat lost from the person's body is replaced by the same amount of heat generated by metabolic activity within the body. For various reasons such as extreme environmental exposure to a cold environment or loss of thermoregulatory ability as a result of disease or anesthesia, a person may develop a body temperature that is below normal, a condition known as hypothermia. A person may develop a condition that is above normothermia, a condition known as hyperthermia, as a result of extreme exposure to a hot environment, or malfunctioning thermoregulatory mechanisms, the latter being a condition sometimes called malignant hyperthermia. The body may also establish a set point temperature (that is, the temperature which the body's thermoregulatory mechanisms function to maintain) that is above normothermia, a condition usually referred to as fever.
Accidental hypothermia is generally a dangerous condition that may even be life threatening, and requires treatment. If severe, for example where the body temperature drops below 30° C., hypothermia may have serious consequences such as cardiac arrhythmias, inability of the blood to clot normally, or interference with normal metabolism. If the period of hypothermia is extensive, the patient may even experience impaired immune response and increased incidence of infection.
Simple methods for treating accidental hypothermia have been known since very early times. Such methods include wrapping the patient in blankets, administering warm fluids by mouth, and immersing the patient in a warm water bath. If the hypothermia is not too severe, these methods may be effective. However, wrapping a patient in a blanket depends on the ability of the patient's own body to generate heat to re-warm the body. Administering warm fluids by mouth relies on the patient's ability to swallow, and is limited in the temperature of the liquid consumed and the amount of fluid that may be administered in a limited period of time. Immersing a patient in warm water is often impractical, particularly if the patient is simultaneously undergoing surgery or some other medical procedure.
More recently, hypothermia may be treated in a more complex fashion. Heated warming blankets may be applied to a patient or warming lamps that apply heat to the skin of the patient may be used. Heat applied to the patient's skin, however, has to transmit through the skin by conduction or radiation which may be slow and inefficient, and the blood flow to the skin may be shut down by the body's thermoregulatory response, and thus, even if the skin is warmed, such mechanisms may be ineffective in providing heat to the core of the patient's body. When breathing gases are administered to a patient, for example a patient under anesthesia, the breathing gases may be warmed. This provides heat relatively fast to the patient, but the amount of heat that can be administered without injuring the patient's lungs is very limited. An alternative method of warming a hypothermic patient involves infusing a hot liquid into the patient via an IV infusion, but this is limited by the amount of liquid that can be infused and the temperature of the liquid.
In extreme situations, a very invasive method may be employed to control hypothermia. Shunts may be placed into the patient to direct blood from the patient through an external machine such as a cardiopulmonary by-pass (CPB) machine which includes a heater. In this way, the blood may be removed from the patient, heated externally, and pumped back into the patient. Such extreme measures have obvious advantages as to effectiveness, but also obvious drawbacks as to invasiveness. The pumping of blood through an external circuit that treats the blood is generally quite damaging to the blood, and the procedure is only possible in a hospital setting with highly trained personnel operating the equipment.
Accidental hyperthermia may also result from various conditions. Where the normal thermoregulatory ability of the body is lost, because of disease or anesthesia, run-away hyperthermia, also known as malignant hyperthermia, may result. The body may also set a higher than normal set point resulting in fever which is a type of hyperthermia Like hypothermia, accidental hyperthermia is a serious condition that may sometimes be fatal. In particular, hyperthermia has been found to be neurodestructive, both in itself or in conjunction with other health problems such as traumatic brain injury or stroke, where a body temperature in excess of normal has been shown to result in dramatically worse outcomes, even death.
As with hypothermia, when the condition is not too severe, simple methods for treating the condition exist, such as cold water baths and cooling blankets, or antipyretic drugs such as aspirin or acetaminophen, and for the more extreme cases, more effective but complex and invasive means such as cooled breathing gases, cold infusions, and blood cooled during CPB also exist. These, however, are subject to the limitations and complications as described above in connection with hypothermia.
Although both hypothermia and hyperthermia may be harmful and require treatment in some cases, in other cases hyperthermia, and especially hypothermia, may be therapeutic or otherwise advantageous, and therefore may be intentionally induced. For example, periods of cardiac arrest or cardiac insufficiency in heart surgery result in insufficient blood to the brain and spinal cord, and thus can produce brain damage or other nerve damage. Hypothermia is recognized in the medical community as an accepted neuroprotectant and therefore a patient is often kept in a state of induced hypothermia. Hypothermia also has similar advantageous protective ability for treating or minimizing the adverse effects of certain neurological diseases or disorders such as head trauma, spinal trauma and hemorrhagic or ischemic stroke. Therefore it is sometimes desirable to induce whole-body or regional hypothermia for the purpose of facilitating or minimizing adverse effects of certain surgical or interventional procedures such as open heart surgery, aneurysm repair surgeries, endovascular aneurysm repair procedures, spinal surgeries, or other surgeries where blood flow to the brain, spinal cord or vital organs may be interrupted or compromised. Hypothermia has even been found to be advantageous to protect cardiac muscle tissue after a myocardial infarct (MI). Controlled reduction in body temperature may also be advantageous in treating and/or preventing other maladies, including ischemic or toxic damage to body tissues and organs, such as, for example, to minimize the toxic effect on the kidneys of contrast agents used during various diagnostic procedures.
Current methods of attempting to induce hypothermia generally involve constant surface cooling, by cooling blanket or by alcohol or ice water rubs. However, such cooling methods are extremely cumbersome, and generally ineffective to cool the body's core. The body's response to cold alcohol or ice water applied to the surface is to shut down the circulation of blood through the capillary beds, and to the surface of the body generally, and thus to prevent the cold surface from cooling the core. If the surface cooling works at all, it does so very slowly. There is also an inability to precisely control the temperature of the patient by this method.
If the patient is in a surgical setting, the patient may be anesthetized and cooled by CPB as described above. Generally, however, this is only available in the most extreme situations involving a full surgical team and full surgical suite, and importantly, is only available for a short period of time because of the damage to the blood caused by pumping. Generally surgeons do not wish to pump the blood for periods longer than 4 hours, and in the case of stroke or traumatic brain damage, it may be desirable to induce hypothermia for longer than a full day. Because of the direct control of the temperature of a large amount of blood, this method allows fairly precise control of the patient's temperature. However, it is this very external manipulation of large amounts of the patient's blood that makes long-term use of this procedure very undesirable.
Means for effectively adding heat to the core of the body that do not involve pumping the blood with an external, mechanical pump have been suggested. For example, a method of treating hypothermia or hyperthermia by means of a heat exchange catheter placed in the bloodstream of a patient was described in U.S. Pat. No. 5,486,208 to Ginsburg, the complete disclosure of which is incorporated herein by reference. Means of controlling the temperature of a patient by controlling such a system is disclosed in U.S. Pat. No. 5,837,003, also to Ginsburg, the complete disclosure of which is incorporated herein by reference. A further system for such controlled intervascular temperature control is disclosed in publication WO 00/10494 to Ginsburg et al., the complete disclosure of which is incorporated herein by reference. Those patents and publication disclose a method of treating or inducing hypothermia by inserting a heat exchange catheter having a heat exchange area into the bloodstream of a patient, and circulating heat exchange fluid through the heat exchange balloon while the balloon is in contact with the blood to add or remove heat from the bloodstream. (As used herein, a balloon is a structure that may be readily inflated by increasing pressure in the balloon and collapsed by reducing pressure in the balloon vacuum.)
A patient's core body temperature can fluctuate unpredictably with the insertion of various medical devices within the patient's body during a medical procedure that can skew the reading of the body temperature when taken in the immediate area of the lumen where the medical device is inserted. Although current medical devices on the market include thermal or temperature sensors mounted directly on the device itself for measurement of the temperature within the body lumen (i.e., a catheter, an electrode on a catheter shaft, etc.), these types of medical devices only measure the temperature of the fluid in the vessel in the immediate area of the inserted device. Further, the placement of the temperature sensor on the catheter used to treat or control the patient's body core temperature puts the sensor in a position where the blood or other body fluid is perturbed by the catheter. For example, cooling or heating fluid flowing through the catheter to a heat exchange device mounted on the distal end of the catheter may slightly heat or cool the body fluid flowing past the body of the catheter upstream of the temperature sensor, resulting in biased temperature readings when the slightly warmed or cooled body fluid reaches the temperature sensor compared to core body temperature as determined by the average blood temperature. Such a bias may result in undershooting the target temperature when the biased temperature readings are used to control heating or cooling of the blood of the patient. This inability to control the patient's body core temperature during a medical procedure because of the devices' difficulty in obtaining an accurate measure of a patient's blood temperature may result in reduced treatment effectiveness if the patient's core temperature is heated or cooled beyond a target temperature.
Although heat exchange catheters, such as described above, provide a rapid and effective means to add or remove heat to a patient's blood to control the body temperature of the patient, accurate control of the temperature of the heat exchange fluid circulating within the heat exchange catheter is necessary to prevent too rapid heating or cooling, or over or under shooting of the target patient temperature sought to be obtained. Various attempts to measure the patient's body temperature during the heat exchange procedure have been attempted. For example, in one method, a temperature probe is inserted in the patient's esophagus and the signal from the temperature probe is communicated to a controller which adjusts the energy being added to or withdrawn from the heat exchange fluid circulating within the heat exchange catheter accordingly. While the esophageal temperature obtained is typically a reasonably accurate measurement of the patient's core temperature, inaccuracies may occur due to improper placement of the probe. Further, placement of the esophageal temperature probe is time consuming, requires precision in placing the probe in the proper area of the esophagus, and also may interfere with other tubes or catheters that may need to be inserted either through the patient's mouth or nasal passage.
Temperature probes, such as thermistors or thermocouples, have been located within the heat exchange catheter itself to provide a temperature signal to the controller. In this method, it is necessary to periodically stop the flow of fluid through the catheter so that the fluid temperature may equilibrate with the temperature of the blood flowing outside of the catheter. Various methods of reducing the amount of time the fluid flow is stopped have also been attempted so that the fluid stoppage does not adversely affect the targeted rate of cooling or heating, nor allow the natural heating of the body to occur which would negate the desired benefit of the induced hypothermia.
One such apparatus and method is described in publication WO 03/015673, entitled “System and Method for Patient Temperature Control Employing Temperature Projection Algorithm”, the disclosure of which is incorporated herein by reference in its entirety. A principle disadvantage of this method is that each time the flow is stopped, the maximum heating or cooling rate is decreased. Moreover, if the interval before the first stoppage is lengthened to speed heating or cooling, the method provides increased risk of overcooling or overheating unless the pump is stopped and the patient's temperature is confirmed. Additionally, when using algorithms to project the actual blood temperature, the fluid flow may never be stopped long enough for the heat exchange fluid to equilibrate with the actual blood temperature, thus providing only an estimate, and not an actual measurement, of the blood temperature.
Another method used has been to locate the temperature probe on the exterior surface of the heat exchange catheter, typically slightly distal to the heat exchange balloon. Such arrangements, however, typically provide fluctuating temperature signals to the controller, which may adversely affect the controller's ability to accurately determine the temperature of the patient's blood. The fluctuating signal is a result of the placement of the temperature sensor in the blood stream. As the blood flows around the heat exchange catheter, the flow of blood tends to separate into a cooler layer immediately adjacent the catheter and a warmer layer further away from the catheter. The situation is reversed if the catheter is being used to warm the patient. As the blood mixes as it flows downstream, the temperature sensor may be exposed to temperature fluctuations caused by incomplete mixing of the blood, which are detected by the sensors, resulting in a fluctuating temperature signal.
For the foregoing reasons, there is a need for an improved heat exchange system that provides for more accurate temperature measurement for use in controlling a heating/cooling means that warms or chills fluid that is then circulated through a heat exchange catheter. Such a system should be capable of estimating the actual core temperature of a patient's body from direct temperature measurement of the patient's blood, and should also be capable of identifying events, such as a change in heating or cooling parameters, a loss of a supplemental warming device, such as a heating blanket, or onset of shivering by the patient that may affect the control of the heating or cooling of the patient. Moreover, such a system should be capable of achieving such an estimate while minimizing or eliminating the interruption of fluid flow through the heat exchange catheter. The present invention fulfills these needs and others.